The molding of hollow articles by solidification of a fluid material against the inner surface of a hollow mold has been practiced for a great many years with a wide variety of materials, including fused solids such as metals or wax which are solidified by chilling the molds, or heat-reactive materials such as heat-denaturable proteins or thermosetting plastics which are solidified by heating the molds, or materials which are caused to change state to a solid by successive heating and cooling such as vinyl plastisols or fluent thermoplastic powders, or even materials which are caused to change state by diffusion of a solidifying ingredient such as latex set up by diffusion of a coagulant from the surface of the mold.
With all of these materials, several manipulative procedures may be used. In slush molding, the mold is completely filled and after a sufficiently thick deposit is formed on its inner surface the excess material is poured out. In rotational casting, the exact quantity of material needed is placed in the mold which is then gyrated to distribute the material uniformly over the inner surface, which is generally accomplished by complex machines which rotate the molds about two axes. In centrifugal casting, the exact quantity of material needed is placed in a generally cylindrical mold which is spun at high speed to cause centrifugal force to distribute the material in such a way as to present a cylindrical free surface on the inside of the article.
If an irregular or non-uniform surface is needed, more or less complex auxiliary mechanisms or procedures are required in all of these previously known processes. Moreover, hollow toroidal objects cannot be produced at all in any simple embodiment of the foregoing procedures.